Pattern generation system using a spatial light modulator

ABSTRACT

A system for creating a pattern on a workpiece sensitive to light radiation, such as a photomask, a display panel or a microoptical device, comprising a source for emitting light pulses in the wavelength range from EUV to IR, a spatial light modulator (SLM) having at least one modulating clement (pixel), adapted to being illuminated by at least one emitted light pulse and a projection system creating an image of the modulator on the workpiece. Further, the system comprises a fast pulse detector to detect an output pulse energy of each individual pulse and produce for each said individual pulse, a signal corresponding to the output pulse energy of said individual pulse, a switch having response times in the nanosecond or sub-nanosecond range for blocking portions of each pulse, said switch being configured to be controlled by said signals from said last pulse detector, so as to control the energy output of each individual pulse to approximately a desired energy output based on the output pulse energy measurement of said individual pulse.

FIELD OF THE INVENTION

[0001] The present invention relates to printing of patterns withextremely high precision on photosensitive surfaces, such as photomasksfor semiconductor devices and displays. More specifically the inventionrelates to a system for creating a pattern on a workpiece comprising asource for emitting light pulses in the wavelength range from EUV to IR,a spatial light modulator (SLM) having at least one modulating element(pixel), adapted to being illuminated by at least one emitted lightpulse and a projection system creating an image of the modulator on theworkpiece.

BACKGROUND OF THE INVENTION

[0002] It is previously known, e.g. from WO 99/45439 by the sameapplicant, to use a spatial light modulator (SLM) in a patterngenerator. This has a number of advantages compared to the morewide-spread method of using scanning laser spots: the SLM is a massivelyparallel device and the number of pixels that can be written per secondis extremely high. The optical system is also simpler in the sense thatthe illumination of the SLM is non-critical, while in a laser scannerthe entire beam path has to be built with high precision. Compared tosome types of scanners, in particular electro-optic and acousto-opticones, the micromirror SLM can be used at shorter wavelengths since it isa purely reflective device. Such a pattern generator comprises a sourcefor emitting light pulses, an SLM with modulating elements (pixels),adapted to being illuminated by the emitted light pulses and aprojection system creating an image of the modulator on the workpiece.

[0003] However, a problem with using SLM in pattern generators is thatfor practical reasons each feature on the workpiece has to be producedby one or at least very few light pulses. Consequently, the systembecomes very sensitive to flash-to-flash energy variations and timejitter. These problems are especially important in gas discharge lasers,such as in a normally used excimer laser. A conventional excimer laserhas flash-to-flash energy variations of 5% and flash-to-flash timejitter of 100 ns. These variations are due to various factors such asvariations in the gain medium and variations in the electrical dischargeprocess. The duration of the laser pulse for a typical excimer laserused for lithography is about 10-20 ns and the pulse frequency is in therange of about 1,000 Hz. By using two exposures for each feature on theworkpiece this problem could to some extent be alleviated, but nottotally eliminated.

[0004] In conventional microlithography, i.e. wafer steppers or waferscanners, using light flashes, such as in integrated circuitlithography, less precision of the light pulses is required, since eachfeature on the workpiece could normally be created by 50 light pulses ormore. Consequently, the integrated exposure on each part of theworkpiece area becomes less sensitive to light pulse variations.However, even in this case flash-to-flash variations are troublesome. Tothis end it is proposed in U.S. Pat. No. 5,852,621 to control the laserpulse energy by using a fast pulse energy detector having response timein the nanosecond or sub-nanosecond range providing an electrical signalrepresenting pulse energy to a trigger circuit. The trigger circuitintegrates the signal and triggers an electro-optic switch, such as aPockels cell, when the integrated signal reaches a predetermined level.The operation of the electro-optic switch trims a portion of the pulseenergy so that the resulting pulse energy is maintained at a consistentlevel.

SUMMARY OF THE INVENTION

[0005] It is therefore an object of the present invention to provide animproved SLM pattern generator for printing of precision patterns.

[0006] This object is achieved with a system according to the appendedclaims.

[0007] It should be noted that the invention specifically relates toprinting of photomasks for semiconductor devices and displays, but alsorelates to direct writing of semiconductor device patterns, displaypanels, integrated optical devices and electronic interconnectstructures. Furthermore, it can have applications to other types ofprecision printing such as security printing. The term printing shouldbe understood in a broad sense, meaning exposure of photoresist andphotographic emulsion, but also the action of light on other lightsensitive media such as dry-process paper, by ablation or chemicalprocesses activated by light or heat. Light is not limited to meanvisible light, but a wide range of wavelengths from infrared (IR) toextreme UV.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] For exemplifying purposes, the invention will be described incloser detail in the following with reference to embodiments thereofillustrated in the attached drawings, wherein:

[0009]FIG. 1 is a schematic illustration of a pattern generator using anSLM according to invention;

[0010]FIG. 2a and b is a schematic representation of a micro mechanicalmodulator according to an embodiment of the invention, illustrating twodifferent working conditions; and

[0011]FIG. 3a-d is an illustration of different preferred arrangementsof stripe formed micro mechanical modulators are illustrated.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0012] Referring to FIG. 1, a pattern generator according to theinvention comprises an SLM 1, preferably with individual and multi-valuepixel addressing, an illumination source 2, an imaging optical system 3,and a hardware and software data handling system 4 for the SLM. Further,the system preferably comprises a fine positioning substrate stage 5with an interferometer position control system 6 or the like.

[0013] The SLM 1 can be built either with micromachined mirrors, socalled micromirrors, or with a continuous mirror surface on a supportingsubstrate so that it is possible to deform using an electronic signal.Other arrangements are however possible as well, such as transmissive orreflecting SLMs relying on LCD crystals or electrooptical materials astheir modulation mechanism, or micromechanical SLMs using piezoelectricor electrostrictive actuation.

[0014] For light in the EUV range a Bragg mirror with deformable mirrorelements or micro mechanical shutters could be used as an SLM.

[0015] The illumination in the pattern generator is preferably done witha KrF excimer laser giving a 10-20 nanoseconds long light flash in theUV region at 248 nanometer wavelength with a bandwidth corresponding tothe natural linewidth of an excimer laser. In order to avoid patterndistortion on the substrate, the light from the excimer laser isuniformly distributed over the SLM surface and the light has a shortenough coherence length not to produce laser speckle on the substrate. Abeam scrambler is preferably used to achieve these two aims.

[0016] Preferably, the pattern generator has a fine positioningsubstrate stage with an irterferometer position control system. In onedirection, y, the servo system keeps the stage in fixed position and inthe other direction, x, the stage moves with continues speed. Theinterferometer position measuring system is used in the x-direction totrigger the exposure laser flashes to give uniform position between eachimage of the SLM on the substrate. When a full row of SLM images areexposed on the substrate the stage moves back to the original positionin the x direction and moves one SLM image increment in the y directionto expose another row of SLM images on the substrate. This procedure isrepeated until the entire substrate is exposed. The surface ispreferably written in several passes in order to average out errors.

[0017] Further, the system according to the invention comprises a fastpulse detector 7 to detect an output pulse energy of each individualpulse and produce for each said individual pulse, a signal correspondingto the output pulse energy of said individual pulse. The detector isconnected to a switch 8 having response times in the nanosecond orsub-nanosecond range for blocking portions of each pulse, said switchbeing configured to be controlled by said signals from said fast pulsedetector. Hereby, the energy output of each individual pulse could becontrolled to approximately a desired energy output based on the outputpulse energy measurement of said individual pulse. Preferably abeam-splitter 12 is arranged ahead of the photo-detector 7, whereby onlya divided part of the beam is detected. The switch could then preferablybe arranged after the beams has been rejoined again, but it is alsopossible to arrange the switch so that it only affects a divide part ofthe beam.

[0018] The switch in the system can for example be a micro-mechanicalmodulator comprising several small reflecting mirrors. The mirrors canbe electrically manipulated to reflect or diffract incoming light indifferent directions depending on an electrical voltage applied to theindividual mirrors. Such a mirror element could e.g. comprise areflective membrane 20 with an underlying electrode 21, such as isillustrated in FIG. 2a. When a voltage is applied between the membraneand the electrode the membrane is drawn towards the electrode, such asis illustrated in FIG. 2b. By the appropriate choice of membrane shapeand electrical addressing of the membranes—in relation to each other orindividually—a diffracting surface can be formed by the modulator.

[0019] The membranes could for example have a stripe like shape. Whenall membranes are flat, i.e. when no voltage is applied to them, theyform a flat reflecting surface. The modulator is arranged so that theincoming light is reflected off the flat surface into a directioncoinciding with the optical axis of the following optical system. If avoltage is applied to every second membrane stripe a binary reflectiongrating is created. The grating diffracts the incoming light into firstand higher diffraction orders. The angle of the first diffraction ordersdepends on the pitch of the grating and is chose so that the first andhigher diffraction orders are not transmitted through the apertures ofthe following optical system. Thus, the modulator works like a lightswitch. This kind of devices are for example manufactured by SiliconLight Machines, having a switch time of about 10 nanoseconds. If therelief height of the diffracting structure is a modulus n of half thewavelength of the incoming light, n being an integer, no light iscontained in the non-diffracted zeroth order, the switch thus blockingall incoming light from entering the following optical system.

[0020] The micro-mechanical modulator could be subdivided into smallersub-areas, each sub-area being comprised of parallel mirror membranestripes and each sub-area constituting a switch. The sub-areas could bearranged in different configurations related to each other, having allthe membranes in parallel or rotated in relation to the other sub-areas.The sub-areas are controlled individually, thus making it possible todiffract part of the beam while letting the rest of the beam passundeflected. Thus, the total transmission of the modulator can be set inan analog fashion, even with binary (on/off) driving of each element. Tocut off the beam completely, all the sub-areas are addressed and arethus made diffracting.

[0021] It is also possible to make each membrane a diffractingstructure. Since just the middle portion of the membrane is attracted tothe electrode, each membrane creates a periodic structure in itself. Byproper design of the membranes, a surface filling array can be created(see FIG. 3a-d), which creates a dark surface when all elements areaddressed. Thus, the transmission of the modulator could be set by therelative number of addressed membranes or membrane cluster.

[0022] Another way of controlling the relative amount of undiffracted todiffracted light would be to control the relief depth of the grating.This could be done if the depth to which each membrane is drawn by theattraction to the electrode could be made continuously dependent on theapplied voltage. Thus, the grating relief height would be controlled bythe value of the applied voltage, the voltage value thus controlling theamount of light in the zeroth order in relation to the first and higherdiffraction orders.

[0023] In FIG. 3a a first preferred arrangement of elongate micromechanical modulators is illustrated. In this embodiment the elongatedmodulators are arranged side by side in columns. In FIG. 3b a secondpreferred arrangement of elongate micro mechanical modulators isillustrated. In this embodiment the elongated modulators are arranged ingroups of parallel modulators, but the groups being arranged inorthogonal relation to each other. In FIG. 3c a third preferredarrangement of elongate micro mechanical modulators is illustrated. Inthis embodiment the elongated modulators are arranged in groups of twomodulators being perpendicular arranged in an angle, and several suchgroups being arranged displaced adjacent to each other. In FIG. 3d afourth preferred arrangement of elongate micro mechanical modulators isillustrated. In this embodiment the elongated modulators are arranged inparallel lines of modulators, being arranged after each other in thelength direction, but the lines being displaced relative to each other.

[0024] However, the switch could also be an electro-optic switch 8, suchas a Pockels cell. In this case the output of the photo detector 7 isfed to an electro-optic cell trigger which integrates the signal andcompares the integrated signal to a predetermined cutoff valuecorresponding to the desired pulse energy. When the cutoff value isreached the trigger produces a trigger signal which activates a highvoltage in the Pockels cell 3. One such suitable Pockels cell isavailable from Energy Compression Corporation, USA, or Gsänger, Germany.Application of high voltage to Pockels cell shifts the polarization oflaser beam by up to 90 degrees so that part of the beam traversing thePockels cell could be rejected by a polarising filter.

[0025] The system according to the invention increases the pulse topulse stability of the pulse energy by cutting the pulse at a presetvalue of integrated pulse energy. The means for cutting the pulse is avery fast switch, e.g. an electro optic switch, such as a Pockels cell,or a micro mechanical modulator as described above. If the level ofpulse energy for which the system is set to cut off the pulse ischanged, the switch will obviously cut the pulse a little earlier or alittle later, depending on the change of pulse energy setting.Considering the very short pulse duration of about 10-20 nanoseconds, itmay be difficult to change the time of clipping considerably due to thespeed of the switch. Thus, the system would work with the same timeconstants regardless of selected transmitted energy dose. Thetransmission of the switch can be changed continuously both for Pockelscell switches and for switches using the micro mechanical modulatordescribed above.

[0026] The transmission of a Pockels cell switch could be controlled bythe polarization of the incident light. It could also be controlled bythe voltage applied to the Pockels cell, thus controlling the amount ofpolarization rotation of the cell and adjusting the amount of lighttransmitted by the second polarizer of the Pockels cell switch.Considering the high power used in Pockels cells it might be convenientto use two Pockels cell switches in conjunction, the first for on/offclipping and the other for smaller transmission modulation.

[0027] To facilitate the clipping of the very short pulse, a pulsestretcher 9 could be used before the detector. Thus, the pulse durationwould be increased making the limited switch time of the following pulsetrimmer less critical and allowing for more accurate clipping of thepulse.

[0028] The pulse stretcher 9 could e.g. comprise two parallel mirrorswith reflection coefficients less than one. The two mirrors thenconstitute an optical resonator. The light is coupled in through thefirst mirror. The limited transmittance of the mirrors causes the lightto make several round-trips within the resonator, some of the lightpassing through the second mirror at each passage. The pulse duration isthus increased. The pulse duration will depend on the photon lifetime ofthe resonator. The photon lifetime depends on the reflectivity of themirrors and the resonator length.

[0029] In order to have enough time to detect the pulse energy and tocontrol the switch accordingly it may be necessary to use a delay unit10, placed between the detector and the switch, to delay the pulse. Thiscould be achieved by arranging reflectors in the beam path to increasethe path length. An increased path length of 1 meter results in a delayof the pulse of about 3-4 ns. The delay time is preferably alsoadjustable. However, other types of delay means could be used as well.

[0030] For high precision pattern generation systems using SLMs, therequired average beam pulse power is very low, normally about 10-100 mW,to be compared to the normal pulse energy power used inmicrolithography, which is normally in the range 10-20 W. A problem withusing such high beam power is that the components being placed after thelaser are exposed to high pulse energies and likely to be damaged. Inthe inventive system it therefore preferred to use a light source withrestricted output power. Alternatively, a conventional laser or the likecould be used, whereby an attenuator 11 could be placed ahead of thepulse detector, for attenuating the pulse energy of each individualpulse.

[0031] Time jitter may also be controlled by the inventive system bycutting both the pulse ends in a controlled manner, leaving a preciselycontrolled middle part. To this end it is possible to use two switches,of the type described above, in series; one to be opened a controlledtime period after the emission of the pulse, and another to be closed acontrolled time period after the opening of the switch.

[0032] Although the pattern generation system above has been describedwith reference to several particular embodiments, it is to beappreciated that various adaptations and modifications may be made. Forexample, functionally equivalent components could be used to replaceseveral of the above described components of the system, such as othertypes of lasers or light sources, such as flash discharge tubes or, forEUVW plasma light sources, other types of delay circuits, other types ofswitching means, other types of detectors, such as photo multiplicatorsetc. Therefore, the invention is only to be limited by the appendedclaims and their legal equivalents.

1. A system for creating a pattern on a workpiece sensitive to lightradiation, such as a photomask, a display panel or a microopticaldevice, comprising a source for emitting light pulses in the wavelengthrange from EUV to IR, a spatial light modulator (SLM) having at leastone modulating element (pixel), adapted to being illuminated by at leastone emitted light pulse and a projection system creating an image of themodulator on the workpiece, characterized in that it further comprises afast pulse detector to detect an output pulse energy of each individualpulse and produce for each said individual pulse, a signal correspondingto the output pulse energy of said individual pulse, a switch havingresponse times in the nanosecond or sub-nanosecond range for blockingportions of each pulse, said switch being configured to be controlled bysaid signals from said fast pulse detector, so as to control the energyoutput of each individual pulse to approximately a desired energy outputbased on the output pulse energy measurement of said individual pulse.2. A system according to claim 1, wherein said switch is controlled toreduce the pulse to pulse jitter and enhance the pulse to pulsestability of the emitted light pulses.
 3. A system according to claim 1or 2, wherein said switch is further controlled in accordance with therequired illumination on the workpiece.
 4. A system according to any oneof the preceding claims, wherein it further comprises a beam splitterfor dividing the light pulse in at least two parts, and the switch beingadapted to control the energy output of only one of those parts, and arejoining unit for subsequently rejoining the pulse parts.
 5. A systemaccording to any one of the preceding claims, wherein said light pulsesource is a laser, and preferably an excimer laser.
 6. A systemaccording to any one of the preceding claims, wherein said fast pulseenergy detector is a photo-diode.
 7. A system according to any one ofthe preceding claims, wherein said switch is an electro optic switch,and preferably comprising a Pockels cell and at least one polarizingbeam splitter.
 8. A system according to any one of the claims 1-6,wherein said switch comprises a micro mechanical modulator.
 9. A systemaccording to claim 8, wherein the micro mechanical modulator comprisesan array of controllable reflecting elements.
 10. A system according toclaim 9, wherein the reflecting elements comprises an elongatereflecting surface.
 11. A system according to claim 10, wherein thereflecting elements are arranged with essentially parallel elongationdirections.
 12. A system according to claim 10, wherein the reflectingelements are arranged with at least two different elongation directions,said two elongation directions having an angle there between, andpreferably being essentially orthogonal.
 13. A system according to anyone of the claims 9-12, wherein the reflecting elements are electricallycontrollable.
 14. A system according to any one of the claims 8-13,wherein the micro mechanical modulator comprises at least onedisplaceable reflective membrane.
 15. A system according to any one ofthe preceding claims, further comprising an optical delay unit.
 16. Asystem according to any one of the preceding claims, wherein the switchis configured to cut the pulse at a desired value of the integratedpulse energy.
 17. A system according to any one of the claims 1-15,wherein the switch is configured to alter the transmission properties tobe used during each individual pulse.
 18. A system according to any oneof the preceding claims, wherein it further comprises a pulse stretcher,being placed ahead of the pulse switch, for extending the pulse durationof each individual pulse.
 19. A system according to any one of thepreceding claims, wherein each written feature on the workpiece iswritten as a superposition of a small number of projected images, andeach image is created by less than four and preferably one light pulses.20. A system according to any of the preceding claims, wherein thespatial modulator is a two-dimensional array of modulating elements withtime-multiplexed loading of the values to the modulating elements andstorage of the loaded value at each element.
 21. A system according toany one of the preceding claims, wherein the spatial light modulatorcomprises an array of micromechanical elements, and preferably an arrayof micromirrors.
 22. A system according to any one of the precedingclaims, wherein it further comprises a pulse energy attenuator beingplaced ahead of the pulse detector, for attenuating the pulse energy ofeach individual pulse.
 23. A system according to any one of thepreceding claims, comprising a second switch controlled to be initiallyclosed and to be opened a controlled time period after the emission ofthe pulse in order to provide a precise control of the starting end ofthe pulse.